What Size Boiler Is Needed Per Square Foot of a House?

It's extremely important to get the exact right size of boiler for your home. Too small a boiler provides insufficient heat to the home, while too large a boiler wastes heat by creating more hot air than your home can handle. Determining the right size boiler for your home comes down to knowing how much power you need per square foot. Factors affecting size requirements include home insulation levels and materials used in the construction of your house.

Home hot water heater and gas boiler
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Contact a manufacturer for size specifications on a boiler.

Boiler Size

Boiler size describes the heat output of a boiler in energy units, not the physical size of the boiler. Manufacturers measure boiler output in British thermal units (BTUs) per hour or boiler horsepower (BHP). BHP measurements usually only apply to very powerful and large capacity boilers, since one BHP equals 33,475 BTUs per hour. A British thermal unit constitutes the amount of power required to increase the temperature of 1 lb. of water by 1 degree Fahrenheit.

The General Rule

According to Alliant Energy, a Midwest energy delivery company, the general rule of thumb holds that indoor spaces require one boiler horsepower of energy per 1000 square feet of space. Remember that one BHP equals 33,475 British thermal units. Therefore, dividing this by 1000, this general rule requires 33.475 BTUs of power output per square foot. Take caution not to purchase a boiler based on this general rule, as numerous factors unaccounted for by this figure affect how much power you actually need from a boiler.

Heat Loss

Heat loss describes the amount of heat your home loses. The heat loss value of your home drastically affects the size boiler you need per square foot. For instance, assume your home contains excellent insulation and airflow control. Such a home requires a much smaller boiler per square foot that an old wood-frame house with no insulation. Numerous factors affect heat loss, and calculating these factors on your own may lead to an improperly sized boiler in your home.

Calculating Your Exact Needs

HVAC professionals use a method called Manual J Load to calculate the exact boiler needs for a space. This scientific method requires assigning values to factors such as home construction materials, insulation levels, size of rooms, number of windows and creating an overall value for your home based on this. You can purchase software to walk you through the Manual J Load process and create values for you, or you can use free online calculators to help determine your needs.